The present invention relates generally to reversed bandgap voltage reference circuits, and more particularly to reversed bandgap voltage reference circuits which are capable of operating from power supply voltages of less than 1 volt and also are more accurate than those of the prior art.
Traditional bandgap voltage reference circuits (see D. A. Johns and K. Martin, “Analog Integrated Circuit Design”, Wiley, 1997, page 357) provides Vout approximately equal to 1.2 volts with less than 50 ppm/degrees Centigrade temperature coefficient (TC) and better than 2% scattering of the output voltage in production volumes without the need for trimming of resistor values. Since the output bandgap voltage is approximately 1.2 volts, the disclosed bandgap reference voltage circuit can not operate from supply voltages below approximately 1.3V.
FIG. 1 illustrates a conventional bandgap reference voltage circuit which includes a PNP transistor Q1 having its base and collector connected to ground, with its emitter receiving a current from a P-channel transistor 6 of a current mirror circuit also including P-channel transistors 5 and 7. PNP transistor Q2 has an emitter area that is N times greater than that of transistor Q1. The base and emitter of transistor Q2 are connected to ground, and its emitter is coupled by a resistor 4 to receive a current from P-channel current mirror transistor 7 equal to the current supplied by transistor 6 to transistor Q1. The emitter of transistor Q1 is connected to the (−) input of operational amplifier 1, and the upper terminal of resistor 4 is connected to the (+) input of operational amplifier 1. The difference in the base-emitter voltages VBE1 and VBE2 of transistors Q1 and Q2, respectively, due to the scaling ratio N of their emitter areas is equal to the voltage across resistor 4 and is used to generate the output bandgap reference voltage Vref. The circuit of FIG. 1 is capable of operation from a 1 volt power supply. A relatively large amount of 1/f noise is generated in the current mirror transistors is in the feedback loop and therefore causes large corresponding noise errors in the generated output voltage Vref. Consequently, a large external filter capacitance is needed to limit the noise bandwidth.
To satisfy the growing need for reference voltage circuits capable of functioning at lower supply voltages, various other attempts have been made to create circuits based on temperature properties of the threshold VTH and carrier mobility μ of the MOS transistor. (See I. M. Filanovsky, A. Allam, “Mutual Compensation of Mobility and Threshold Voltage Temperature Effects with Applications in CMOS Circuits”, IEEE TCAS-I, vol. 48, no. 7, pp. 876-884, 2001). However, relatively poor manufacturing repeatability and poor control of the process-defined VGS threshold voltage VTH prevents the circuits disclosed in these references from being widely adopted by the industry. Accuracy and production “scattering” of such bandgap reference voltage circuits are significantly worse than for the traditional bandgap reference voltage circuit shown in FIG. 1.
FIG. 2 shows a known current mode bandgap reference voltage circuit which includes an NPN transistor Q1 having its emitter connected to ground, its base connected to one terminal of a resistor R1 having its other terminal connected to ground. The collector of transistor Q1 is connected to Vref and to the emitter of a diode-connected NPN transistor Q2 having an emitter area that is N times that of transistor Q1. The base and collector of transistor Q2 are connected to one terminal of a current source and to one terminal of a resistor R2, the other terminal of which is connected to the base of transistor Q1. This circuit has the shortcomings that transistor Q1 operates close to saturation and therefore the circuit is subject to errors caused by large base currents. This circuit also requires the current source Ibias to be a complicated circuit capable of providing a complicated temperature coefficient.
Various other attempts also have been made to create bandgap reference voltage circuits based on current-mode operation, by combining positive-TC and negative-TC current sources to create a temperature independent current. This current is transferred to a resistor by a current mirror to generate the reference voltage. (See P. Malcovati, F. Maloberti, C. Fiocci, and M. Pruzzi, “Curvature-compensated BiCMOS bandgap with 1-V supply voltage”, IEEE JSSC, vol. 36, no. 7, pp. 1076-1081, 2001). However, the main drawback of the current-mode reference voltage circuits disclosed in these references is the presence of the current mirror. Regardless of the circuit techniques and components used to create the positive-TC and negative-TC currents, the accuracy of such reference can not be better than accuracy of the current mirror and the resistor (considering matching and noise). In general, an improvement in current mirror accuracy can be achieved with sampling techniques. The noise can be reduced by using a large filtering capacitor at the output. However, this leads to a more complicated circuit, larger die area and increased current consumption, with somewhat compromised accuracy.
An attempt been made to use what is referred to herein as a “reversed bandgap principle”, using NPN transistors in FIG. 2 of “A CMOS Bandgap Reference Circuit with Sub-1-V Operation” by H. Banba, H. Shiga, A. Umezawa, T. Miyaba, T. Tanzawa, S. Atsumi, and K. Sakui, IEEE JSSC, vol. 34, no. 5, pages 670-674, 1999. Also see FIG. 4 of “Low Voltage Techniques” by R. J. Widlar, IEEE JSSC, vol. 13, no. 6, pages 838-846, 1978.
The voltage reference in FIG. 2 of the Banba reference using the “reversed bandgap principle” has been implemented with NPN transistors. One of the core NPN transistors operates with ˜190 millivolts collector-to-emitter voltage (VCE). Being that close to saturation, the parasitic substrate PNP structure, which is present in vertical NPN transistors on all but SOI (silicon on insulator) processes, becomes activated. This in turn increases the value of the base current and decreases its predictability. This circuit also requires a separate bias with a complicated TC. As a result, the accuracy is poor and this reference voltage circuit cannot compete with traditional bandgap reference voltage circuits at higher supply voltages.
Thus, there is an unmet need for a reversed bandgap voltage reference circuit which provides a more precise reference voltage than has been previously obtainable from reference voltage circuits capable of operating from power supply voltages of less than 1 volt.
There also is an unmet need for a reversed bandgap voltage reference circuit which provides a more precise reference voltage having substantially lower noise than has been previously obtainable from reference voltage circuits capable of operating from power supply voltages of less than 1 volt.
There also is an unmet need for a reversed bandgap voltage reference circuit which provides a more precise reference voltage having substantially lower noise than has been previously obtainable from reference voltage circuits capable of operating from power supply voltages of less than 1 volt and which avoids the need for providing complex current source circuitry to compensate for temperature coefficient errors.